For millions of years, green plants have employed photosynthesis to capture energy from sunlight and convert it into electrochemical energy. Scientists around the world are in pursuit to develop an artificial version of photosynthesis that can be used to produce fuels from carbon dioxide and water.
Increasing concentrations of greenhouse gases produced by human activities such as deforestation, burning fossil fuels and industrial revolution have contributed to global warming. Global warming is showing adverse effect on ecological, social systems and solutions are being sought by researchers to reduce the greenhouse gases and thus addressing to the problems of global warming.
One of the solutions for global warming is experimented on the usage of non-fossil fuels and production of non-fossil fuel without the emission of carbon dioxide. Hydrogen is the major non-fossil fuel which can be produced by electrochemical splitting of water without the emission of carbon dioxide. Therefore, formulation of a new electrocatalyst for electrochemical water splitting giving rise to hydrogen and oxygen is of prime interest.
Electrolytic gas production involves transfer of four protons and four electrons with the formation of an oxygen-oxygen bond at the anode concomitant with reduction of protons to produce hydrogen at the cathode. Oxygen evolution is a difficult multistep four electron transfer reaction and usually it requires higher overpotential.
Efforts have been made to try to reduce the amount of overpotential needed to drive the reaction by using specialized anodes and/or a catalyst. At present, catalysts based on Ru, Ir and Pt are used for electrochemical splitting of water, which are very costly. Also, catalysts used in prior arts are observed to degrade under reaction conditions. Cobalt oxide materials are known in the art as water-electrolysis catalysts.
An article titled “In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+” by Matthew W. Kanan and Daniel G. Nocera, Vol 321, Science, pg 1072-1074 disclose oxygen-evolving catalyst that forms in situ upon anodic polarization of an inert electrode in neutral aqueous phosphate solutions containing Co2+. Oxygen generation occurs under pH=7, 1 atm, and room temperature. Cobalt ions in the presence of chemical oxidants such as Ru(bpy)3+ (bpy, bipyridine; Eo=1.26, where Eo is the standard potential) catalyze the oxidation of water to O2 in neutral phosphate solutions. A Tafel plot at pH 7 is carried out and it shows overpotential of 410 mV at 1 mA/cm2 current density. Extrapolation of straight line gives exchange current density of 4 to 6×10−11 A/cm2. Moreover, catalyst is synthesized by electrodeposition and it is amorphous in nature. An article titled “Cobalt Oxide Nanocrystals and Artificial Photosynthesis” by Frei and Jiao discloses rod-shaped Co3O4 crystals measuring 8 nanometers in diameter and 50 nanometers in length as photocatalysts in artificial photosynthesis. The nanorods are interconnected by short bridges to form bundled clusters which are shaped like a sphere with a diameter of 35 nanometers.
An article titled “Nanostructured cobalt oxide clusters in Mesoporous silica as efficient Oxygen-Evolving Catalyst” in Angew Chem 2009, 48, 1841-1844 relate to photocatalytic water splitting in the presence of a costly sensitizer (dye-[Ru3+(bpy)3]/[Ru2+(bpy)3]).
In the above mentioned prior arts discloses the use of catalyst for photocatalytic water splitting that contains Hydrogen evolution reaction (HER) and Oxygen evolution reaction (OER). The present invention is proposed to study of kinetics of oxygen evolution reaction in electrochemical method. Further, the prior arts disclose photo electrochemical measurements with sensitizer (dye).
“An article titled “Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes” in International Journal of hydrogen energy, 36 (2011)72-78, reports the exchange current density of 4.7×10−9 A/cm2 for Co3O4 nanowires for oxygen evolution reaction.”
US2011127170 titled “Cobalt Oxyfluoride Catalysts for Electrolytic Dissociation of Water” relates to electrodeposition of cobalt oxyfluoride. The catalyst facilitates the electrolytic conversion of water to hydrogen gas and oxygen gas at neutral pH and at room temperature. Even though it is mentioned in said patent that a neutral pH is employed, most of the experiments are carried out at pH 5. Further, the material is not stable below 1 Volt and dissolution occurs. Moreover, during prolonged electrolysis in cobalt free buffer, decrease in current is observed. To achieve steady state current, 0.1 mM Co2+ or higher concentration of fluoride in electrolyte is needed. Also, Tafel plot and details of overpotential, exchange current density are not disclosed in the patent.
An article titled “Shape-controlled synthesis of porous Co3O4 nanostructures for application in supercapacitors by Ting Zhu, Jun Song Chen and Xiong Wen Lou et. al in J. Mater. Chem., 2010, 20, 7015-7020 reports a method for the shape-controlled synthesis of cobalt carbonate/hydroxide intermediates. Three different structures, viz., one-dimensional (1D) needle-like nanorods; two-dimensional (2D) leaf-like nanosheets, and three-dimensional (3D) oval-shaped microparticles with specific surface areas of 86.1-121.5 m2 g−1, are synthesized through varying experimental parameters such as precursor (cobalt acetate) concentrations and volume ratio of polyethylene glycol to water. The obtained porous Co3O4 find potential application in supercapacitors. The method of synthesis is hydrothermal and application is different from instant application.
U.S. Pat. No. 7,976,989 titled “Precious Metal Oxide Catalyst for Water Electrolysis” disclose a composite catalyst which contains precious and costly metal oxides IrO2 and RuO2 and high surface area inorganic oxides for use as anode catalysts in PEM (polymer electrolyte membrane) water electrolysis. The method disclosed is PEM (Polymer Electrolyte Membrane) electrolyzers for water electrolysis. The method and the composite catalyst are different from the instant invention. Further, there is no mention of overpotential required for water electrolysis.
Article titled “From cobalt nitrate carbonate hydroxide hydrate nanowires to porous Co3O4 nanorods for high performance lithium-ion battery electrodes” by Hui Zhang, Jianbo Wu, et. al in Nanotechnology Volume 19 Number 3, disclose synthesis of cobalt nitrate carbonate hydroxide hydrate (Co(CO3)0.35(NO3)0.2(OH)1.1.1.74H2O) nanowires via the hydrothermal process using sodium hydroxide and formaldehyde as mineralizers at 120° C. The porous Co3O4 nanorods 10-30 nm in diameter and hundreds of nanometers in length have been fabricated from the above-mentioned multicomponent nanowires by calcination at 400° C. The method disclosed in said article is hydrothermal method for synthesis of Co3O4 nanorods. It is a complex high temperature (1200° C.) method. The said article however does not disclose BET surface area.
Article “Int. J. Electrochem. Sci., 7 (2012) 3350-3361” also used Cobalt oxide for water splitting but the material used in the article is CoOx and not Co3O4. These two materials are not same. Moreover in the article, they have synthesized CoOx by electrodeposition. However, we have followed a very simple method to synthesize Co3O4 nanorods.
In the above mentioned article, Tafel plot is not shown. Therefore, for comparison of our Tafel plot in 0.1M KOH with the reported one, we have extracted the data points from the graph in the article and Tafel plot is plotted. Both Tafel plots are compared in the FIG. 1.
FIG. 1 clearly shows that Co3O4 (present work) has less overpotential (399 mV) compared to CoOx (437 mV) at 1 mA/cm2 (−3 on x-axis). In the CoOx Tafel line, after 5.6 mA/cm2 (−2.25 on x-axis), the potential increases very sharply which shows that the point is the limiting current region. After that particular current density, the kinetics of reaction is governed by the mass transfer. So, usually the overpotential value should be reported at particular current density which should be less than the limiting current. In that sense, the overpotential value can be compared at 1 mA/cm2 (−3 on x-axis). So at 1 mA/cm2, Co3O4 has still 38 mV less overpotential compared to that reported for CoOx.
As seen from above, there remains a need to develop a cost effective and efficient catalyst which catalyzes electrochemical splitting of water for production of oxygen and hydrogen at a lower overpotential and in energy efficient manner.